High-temperature superconductors (HTS) have been developed by research institutions and industry, and substantial efforts are currently being made to commercialize this technology. Two large sets of applications are currently driving the commercialization efforts: power generation and transport and high field magnet systems. One example of HTS is Rare Earth Barium Cuprates (REBCO) coated tape, which has demonstrated a great potential for high field magnets by retaining the superconducting state in fields above 100 T at liquid helium temperatures, whereas materials that are currently used to build superconducting magnets, i.e. Nb—Ti and Nb3Sn, can only operate in fields below 25 T.
For technological reasons, HTS materials are commonly manufactured as tape, and therefore, engineering approaches normally applied to round wires are inapplicable to HTS tape. Due to the geometrical and mechanical constraints, tape conductors cannot be easily bent to change direction when entering and exiting the coil section at some pre-determined winding pitch angle. There are two bending modes for a tape conductor with respect to the conductor's broad surface: a soft bend, which is an out of plane bend, and a hard-bend, which is an in-plane bend. Minimum bending radii of soft-bends are on the order of several millimeters and are significantly smaller than minimum bending radii of hard-bends, which are on the order of several centimeters. For this reason, hard-bends result in degradation of the electrical transport properties of the conductor and, therefore, are undesirable.
HTS tape is produced in pieces of relatively short length, thus requiring electrical joints between the pieces. Since currently no technology exists that allows for making of superconducting joints with HTS materials, all electrical joints are resistive and contribute to the total heat generation of the coil. Common joint technologies include various approaches involving overlapping pieces of conductor tape in attempt to achieve sufficiently low contact resistance.
A major problem associated with HTS high field magnets is “trapping” of helium gas. High field magnets utilizing HTS materials are cooled by liquid helium. Due to its low thermal capacity and latent heat of vaporization, liquid helium easily transitions into the gas phase when in contact with a heat source, resulting in drastically reduced cooling of the coil if the gas is not replaced by liquid within that region. A well documented phenomenon is the trapping of helium gas in areas where the magnetic force, which is proportional to the product of magnetic field and field gradient, is at its highest. An effective magnetic moment exists in boiling liquid helium because of the differences in the dielectric moment of helium gas and liquid. This is typically the case at the ends of solenoid coils, where generated heat may cause the helium gas to remain and accumulate.
Coincidentally, the ends of solenoid coils are also the areas where the electrical connections between the coil and the current leads, which connect the coil to a power supply outside of the cryogenic environment, have to be established. Typically, these connections are made in resistive joints located within this critical region at the inner and outer first turn of the coil. These resistive joints can generate a lot of heat, and, due to liquid helium being converted to gas, the joint may become insufficiently cooled, causing the coil to either not perform at its fullest potential or not maintain its superconducting properties and fail.
Accordingly, there is a need for an electromagnetic coil system for use in high field magnets utilizing high-temperature superconductors, where the coil system contains long distributed resistive joints to reduce contact resistance, heat generation, and reduce the helium gas trapping problem.